Noise cancelling magnetic antenna for use with watercraft

ABSTRACT

An antenna for extremely low frequencies (less than 3 KHz) comprising a conductor wound coaxially around a cable for substantially its full length and then wound back on itself in the same sense as the initial forward winding to place both terminals at one end of the antenna. The forward and return helices are in series electrical connection thus providing an increased S/N ratio. The chief advantage of the double helix over the normal helix is that the unbalance due to vibration induced noise is compensated essentially turn-to-turn.

United States Patent [191 Powers et al.

[ 1 Oct. 14, 1975 NOISE CANCELLING MAGNETIC ANTENNA FOR USE WITH WATERCRAFT [75] Inventors: Kerns H. Powers, Princeton, N..l.; Lewis L. Stetz, Jr., Levittown, Pa.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

[22] Filed: Sept. 6, 1974 [21] Appl. No.: 503,582

[52] US. Cl 343/709; 343/895 [51] Int. Cl. H01Q 1/34 [58] Field of Search 343/709, 710, 719, 895

[5 6] References Cited UNITED STATES PATENTS 1,898,661 2/1933 Hagen 343/895 12/1962 Tennyson 343/709 12/ 1971 Fredriksson et al. 343/719 Primary Exar'ninerEli Lieberman Attorney, Agent, or FirmR. S. Sciascia; P. Schneider; W,-T. Ellis 1 57 ABSTRACT An antenna for extremely low frequencies (less than 3 [(1-12) comprising a conductor wound coaxially around a cable for substantially its full length and then wound back on itself in the same sense as the initial forward winding to place both terminals at one end of the antenna. The forward and return helices are in series electrical connection thus providing an increased S/N ratio. The chief advantage of the double helix over the normal helix is that the unbalance due to vibration induced noise is compensated essentially tum-to-turn.

3 Claims, 5 Drawing Figures US. Patent Oct. 14, 1975 Sheet 2 of2 3,913,107

SINGLE HELIX STATIC CONDITION DOUBLE HELIX STATIC CONDITION SINGLE HELIX HALF-WAVE BEND (BEND IS IN 2 DIRECTION) DOUBLE HELIX HALF-WAVE BEND NOISE CANCELLING MAGNETIC ANTENNA FOR I USE WITH WATERCRAFT FIELD OF THE INVENTION The invention relates generally to an antenna operable in the very low frequency range of the electromagnetic energy spectrum and in particular to an inductivetype helix antenna operable in the very low frequency range.

PRIOR ART Radio communication is conducted in a number of frequency bands of the electromagnetic energy spectrum. The lowest of the frequency ranges, commonly designated the extremely low frequency (ELF) range, extends to an upper limit of 3 kilocycles per second. Radio communication in the ELF range possesses several favorable characteristics. Thus the propagation of ground waves is subject to less attenuation, the atmospheric absorption is much less, and the propagation of sky waves is less affected by ionospheric conditions than at higher frequencies. The major advantage to communication in the ELF range is the ability of radio waves at this frequency to penetrate sea water and thereby permit communication with submerged vessels.

A disadvantage of communication at ELF frequencies is that the loop-type antennas normally used for the reception of VLF (3-30 KI-Iz) are extremely susceptible at ELF (below 3KHz) to induced, noise voltages resulting from loop motion in the Earths magnetic field.

For this reason electrode pair antennas have been favored for ELF receiving antennas on submarines. Early submarine ELF antennas therefore consisted of an electrode pair on a long towed cable. But this type of antenna has the disadvantage of having a figure-ofeight, antenna pattern which provides a signal null broadside to the submarine:

The problem of the broadside-null is solved by the use of a helical antenna wrapped around a core. This type of antenna provides a figure-of-eight pattern with the maximum signal lobes broadside to the submarine. A major advantage to this helical winding configuration is that it reduces noise susceptibility. As the cable undergoes undulations and vibrations from water turbulence, the stiffness of the cable leads to quasisinusoidal deflections of the cable with the net result that for .to any desired value by making the helix sufficiently long.

Helix configurations of this type return the end of the helix through the cable via the center conductor. Although this helical configuration balances the noise voltage generated in each twisted loop with an opposite phased noise voltage generated by an oppositely twisted loop, this balancing is only accomplished over a full wavelength of the deformation. This can be seen from an examination of FIG. 2a and FIG. 3a. FIG. 2a is projection on the X-Z plane of a helix antenna in a static condition extending down the X-axis. It can be seen that the helix is completely balanced. FIG. 3a is a projection of the same helix with a half-wave bend in the Z-direction. Due to the movement of the center conductor 24 within the loop projection, areas 26 are clearly larger than the loop projection areas 28. (The noise voltage generated by the movement of a loop in a direction normal to a magnetic field is proportional to the loops projection area in that direction). Thus, since the areas 26 are not completely compensated over a half wavelength, there is a residual unbalance. As can be seen from the dotted extention of the helix, this residual unbalance, is only compensated for over the full wavelength.

This residual unbalance is a disadvantage firstly, because the amount of induced-noise-voltage unbalance is dependent on the length of the helix and the frequency of the. "sinusoidal deformation. (The closer the helix length is to a full wavelength the better the balance).

Secondly, the center wire in combination with the unbalanced helix forms a side-looking loop. Such a loop has the capability of receiving and transmitting electromagnetic flux changes from the side (perpendicular to the helix axis). This is a disadvantage if the transmissions and receptions are desired to be highly directional. Lastly, complete balancing might not even be accomplished with a full-wavelength because this I balancing depends on the uniform sinusoidal deformation of the center conductor. If, due to local forces, the center conductor is moved from the position it is required to take for self-balance over the full wavelength distance, then unbalanced areas may occur. In other words, the forces on the single helix and on the center conductor are not the same, but are only indirectly related, thus providing the conditions for a possible unbalance.

SUMMARY OF THE INVENTION Briefly, the present invention comprises a helix wound around a core in one sense to the end of the core and then wound back on itself along the core in the same sense to the electromagnetic energy field as the initial helical winding, so that both terminals of the antenna sensor are at one end of the core. The two helices are thus connected in series so that the induced signal voltage is twice that of a single helix for a given cable length. The vibration-induced noise voltage, since it rises in the r.m.s. manner, is less than doubled with the result being a marked improvement in the sensitivity. Thus the required length to obtain a proper S/N ratio has been cut down considerably. Furthermore, the turn-by-turn balancing that is effected by this configuration appeciably reduces the noise unbalance at less than full wavelengths and in turn, the possible sidetransmission capability. The fact that both the forward and return helices are wrapped on a common base and thus deform in substantially the same manner removes the possibility of the occurrence of an unbalanced area caused by local forces on the center conductor.

OBJECT OF THE INVENTION An object of the present invention is to reduce the required length of helix needed to obtain a proper S/N ratio for transmission or reception.

A further; object of the present invention is to improve the balance of the helix antenna against vibration induced noise.

A still further object is to reduce the end-effect voltage and side transmission capability of a helical antenna.

Otherobjects, advantages, and novelfeatures of the present invention will become apparent from the fol.- lowing detailed description of the invention when considered in conjunction with the accompanying drawmgs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic broadside representation of the helical antenna of the present invention with an expanded horizontal scale.

FIG. 2a is the projection of an X-axis single helix in the -X-Z plane,

FIG. 2b is the projection of an X-axis double helix in the X-Z plane, A

FIG. 3a is the projection of an X-axis single helix with a half-wave bend in the Z-direction.

FIG. 3b is a projection of the X-axis double helix with a half-wave bend in the Z-direction.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a broadside, expanded, schematic representation of the return-wound helix of the present inven-' tion. The loops forming the forward helical winding begin at the terminal 12. The arrows on the windings shown in the drawing indicate the. winding direction. The forward winding It) continues to the point 14. At the point 14 the winding is reversed and wound back on itself. The loops 16 form the return helical winding, as the loops 10 form the forward helical winding.

The vibration-induced, noise voltage produced by two of, the inner loops is shown by the vectors B and B The vector B represents the vibration-induced, noise voltage generated in the forward-wound loop 18. The dashed line vectors B and E represent the. normal and horizontal components of the vector B,. The vector B represents the vibration-induced, noise voltage generated in the return loop 20. The dashed line vectors, B and B represent the normaland horizontal components of the vector B As can be seen from the figure, the vibration-induced noise voltages resulting from the normal component B of the Earths magnetic field are of equal value but of opposite phase in the forward and return helices. However the noise voltages produced from the in-line componentB are in phase and add linearly. Thus the effect of the return helix is to double that noise arising from the in-line component B while cancelling that noise arising from the normal component B of the field. The total noise voltage is thus less than double.

FIG. 2b shows the projection of the double helix extending down the X-axis on the X-Z plane under static conditions. Clearly the vertical-lined areas 34 are completely cancelled out by the adjacent, oppositely phased, horizontal-lined areas 36. FIG. 3b shows the projection of the same helix on the X-Z plane under a half-wave bend. From an inspection of the drawing, it can be seen that the vertical lines areas 34 are almost completely canceled by the oppositely phased, adjacent, horizontal-lined areas 36. Any small residual unbalance is cancelled out over a half wavelength. This essentially turn-to-turn balancing removes the dependence of the noise balancing on the helix length and the frequency of the sinusoidal deformation.

This type of balancing also reduces the end-effect voltage since there is a much shorter length of unbal-f anced area in the double as opposed to the single helix design. It also substantially reduces the capability of the antenna for side transmissions.

A further improvement in self-balancing can be ex- I pected from the double helix because, since both the forward and the return helix are wrapped on a common base, any deformation of that base cause by bending deforms both helices in substantially the same manner. In the single. helix case, deformation of the helix caused by deformation of the base may develope unbalanced h areas due to local forces on thecenter conductor moving it from the position required for self-balance over 1 i the pertinent distance (a full-wavelength). The double helix eliminates the problem of possible unbalance due to a moving center conductor by eliminating the center conductor in'the helix design.

It should be understood that if there is a full-wave" bend in the helical structure, then the single and the double helix designs both achieve balancing. But the double helix achieves .balance in a half wavelength as opposed to the full wavelength for the single helix.

It should be further understood that this improvement in balancing of the double helix is obtained only i I in one direction the plane formed by the helix axis and the bend direction (the X-Z plane in the example given). The double helix gives no improvement in balancing over the single helix in any other planes. Of

course it should be pointed out that the cable is being;

deformed in three dimensions. Thus the deformation component in each direction has a projection plane in which the double helix provides essentially turn-to-turn balancing whereas the single helix would have provided balancing only over a full-wavelength bend.

The helix may be wound around any type of core.

The core may or may not have magnetic properties. Of course the use of a high-permeability, low-resistivity core material will enhance the effective area of the antenna.

The present embodiment uses a cable with or without a center conductor as the core. This cable is then at-. tached to a nautical vessel and towed through the wa-.

1. An inductive-type antenna system for the transmis sion and reception of electromagnetic radiation in the extremely low frequency range comprising:

flexible core means; and electrical conductor means wound coaxially around said core means along substantially the total length thereof and then wound coaxially around said core V 7 means back on itself in the same sense as the initial winding along substantially the total length of said core means to provide the second terminal to com plete a circuit.

along substantially its entire length and then winding said conductor coaxially back on itself in the same sense as the initial forward winding along substantially the entire length of said cable; and

c. towing this conductor-wound cable from the stern of the nautical vessel through the water. 

1. An inductive-type antenna system for the transmission and reception of electromagnetic radiation in the extremely low frequency range comprising: flexible core means; and electrical conductor means wound coaxially around said core means along substantially the total length thereof and then wound coaxially around said core means back on itself in the same sense as the initial winding along substantially the total length of said core means to provide the second terminal to complete a circuit.
 2. An inductive-type antenna system as defined in claim 1 wherein said core means is a cable.
 3. A method fo receiving extremely low frequency electrical waves and thus allowing efficient underwater communication comprising the steps of: a. attaching a flexible cable to the stern of a nautical vessel; b. winding a conductor coaxially around said cable along substantially its entire length and then winding said conductor coaxially back on itself in the same sense as the initial forward winding along substantially the entire length of said cable; and c. towing this conductor-wound cable from the stern of the nautical vessel through the water. 